Bearings are built as rolling element bearings or as plain bearings. A plain bearing, or sliding bearing, is also known as a hydrodynamic bearing, a hydrostatic bearing or a hybrid bearing.
The contact surface between a rolling element and a bearing shell is very small. In an arrangement, where higher loads act and higher moments have to be transferred over the bearing, rolling element bearings are not sufficient.
In such arrangements, plain bearings are applied. Plain bearing or sliding bearings are capable to transfer higher loads than rolling element bearings.
In a direct driven wind turbine rotor of the electrical generator is directly coupled with the rotor of the wind turbine. The stator of the electrical generator is attached to the stationary part of the wind turbine. Thus the rotor and the stator of the electrical generator are rotatable connected via the main bearing of the wind turbine.
In direct driven wind turbines a main bearing is connecting the rotor of the wind turbine with the stationary part of the wind turbine.
The main bearing of the direct driven wind turbine has to transfer high loads. As wind turbines are increased in size, and also in rotor diameter, sliding bearings are used as a main bearing.
The air gap between the rotor and the stator of the electrical generator depends on the parameters of the bearing. Especially the clearance of the bearing has a high influence on the width of the air gap of the electrical generator.
When the clearance of the main bearing is too high, the tolerance in the air gap is very high. This might lead to a contact between the rotor and the stator and thereby to damages in the electrical generator of the wind turbine.
When the clearance of the main bearing is too low, layer of lubrication might not have a sufficient thickness and the friction in the bearing is too high. This results in damages in the bearing and a reduced live-time of the bearing components.
A too low clearance in the bearing causes a rise in the temperature of the bearing components due to higher friction. In addition it reduces the circulation and exchange of the lubricant in the bearing.
Thus the clearance of the main bearing has to be kept within a certain predetermined range of values to prevent damages.
The clearance of the bearing is highly influenced by the temperature of the bearing components. Most bearing shells are of metal materials, which show a certain expansion at increasing temperatures.
During the start-up of a wind turbine, for example, the lubricant in a sliding bearing is not provided as a film of lubricant. The film of lubricant is built up by the rotation of the bearing. In this case, the friction in the bearing is very high at the start-up of a wind turbine. The friction is generating heat.
The bearing shells in the bearing show a different constructional shape, mass and surface area. Due to this, one of the bearing shells will experience a faster increase in temperature during the start-up of the wind turbine.
This leads to a different expansion of the bearing shells. Due to that the clearance between the bearing shells will change.
To keep the clearance of the bearing within a predetermined range, it is known to measure the temperature of the bearing components. According to the value of the temperature of the bearing components the components are cooled or heated to keep the bearing at a constant operational temperature.
To achieve this, cooling systems, like water cooling or oil cooling systems are used to keep the bearing shells at a certain predetermined temperature.
This shows the disadvantage that the reaction of the cooling system on the temperature measured is slow and that the cooling system consumes a high amount of energy for cooling the bearing shells to a certain predetermined temperature.